The present invention relates to hematite particles aggregates, a non-magnetic undercoat layer for magnetic recording medium using the hematite particles aggregates, and a magnetic recording medium having the non-magnetic undercoat layer, and more particularly, to hematite particles aggregates as non-magnetic particles capable of forming a non-magnetic undercoat layer of a magnetic recording medium having a more excellent surface smoothness, which can exhibit not only an excellent dispersibility but also have an improving ability of a surface smoothness of a coating film produced therefrom by calendering treatment; a non-magnetic undercoat layer for magnetic recording medium which contains the hematite particles aggregates and exhibits a more excellent surface smoothness; and a magnetic recording medium having the non-magnetic undercoat layer, which exhibits a more excellent surface smoothness.
With recent tendency toward long-time recording, miniaturization and weight-reduction of video or audio magnetic recording and reproducing apparatuses, magnetic recording media such as magnetic tapes and magnetic discs have been increasingly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, improved frequency characteristics and a lower noise level.
In particular, video tapes have been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short wavelength region have come to be used and as a result, the magnetization depth from the surface of the magnetic tape has come to be remarkably small.
With respect to short wavelength signals, reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. In order to achieve the reduction in thickness of the magnetic recording layer, it is required to smoothen the surface of the magnetic recording layer and eliminate unevenness in thickness thereof. For this purpose, the base film is also required to have a smooth surface.
In ordinary magnetic recording media, the surface of the magnetic recording layer has been smoothened by forming a magnetic recording layer containing magnetic particles and a binder resin on a non-magnetic base film, and then subjecting the magnetic recording layer to calendering treatment.
In recent years, with further reduction in thickness of the magnetic recording layer, there has been proposed such a method of forming one undercoat layer comprising a binder resin and non-magnetic particles such as acicular hematite particles dispersed therein (hereinafter referred to as xe2x80x9cnon-magnetic undercoat layerxe2x80x9d) on a non-magnetic base film in order to solve problems such as deterioration in surface properties and electromagnetic performance of the magnetic recording layer, and magnetic recording media having such a non-magnetic undercoat layer have been already put into practice (Japanese Patent Publication (KOKOKU) No. 6-93297(1994), and Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993) and 5-182177(1993)).
In the case of such magnetic recording media having the non-magnetic undercoat layer, the non-magnetic undercoat layer comprising non-magnetic particles and a binder resin, and the magnetic recording layer comprising magnetic particles and a binder resin are successively formed on the non-magnetic base film, and then the obtained medium is subjected to calendering treatment to absorb irregularities on the non-magnetic base film by the non-magnetic undercoat layer, thereby smoothening the surface of the magnetic recording layer. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 5-12650(1993), it is described that xe2x80x9c. . . in the case where a non-magnetic layer is provided, when the layer containing hexagonal system ferrite-based magnetic particles is subjected to surface-smoothing treatment, the non-magnetic layer formed immediately beneath the magnetic layer is collapsed as a buffer layer. At this time, the underlying non-magnetic layer acts as an absorbing layer, so that the surface of the upper magnetic recording layer containing hexagonal system ferrite-based magnetic particles can be smoothened . . . xe2x80x9d.
Hitherto, in order to improve properties of magnetic recording media, various attempts for non-magnetic particles used in the non-magnetic undercoat layer have been conducted. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 6-60362(1994), there is described a non-magnetic undercoat layer for magnetic recording media, which contains non-magnetic particles composed of acicular hematite particles coated with an Al compound. Also, in Japanese Patent Application Laid-Open (KOKAI) No. 10-334450(1998), there is described a magnetic recording medium having a non-magnetic undercoat layer containing fine acicular goethite particles in which three or less particles are overlapped at the same crystal planes and adhered with each other along the crystallographical a-axis direction thereof.
At present, it has been strongly required to provide acicular hematite particles as non-magnetic particles capable of forming a non-magnetic undercoat layer for a magnetic recording medium having a more excellent surface smoothness. However, such acicular hematite particles have not been obtained conventionally.
Namely, the hematite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 6-60362(1994) do not have such a structure as oriented in the major axis direction thereof. Therefore, it is difficult to improve a surface smoothness of a coating film produced using such hematite particles by calendering treatment.
Also, even though the goethite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 10-334450(1998) are used as non-magnetic particles for non-magnetic undercoat layer, it is difficult to attain the aimed dispersibility because of poor compatibility with binder resins or solvents due to a large amount of crystal water contained in the goethite particles.
As a result of the present inventors earnest studies, it has been found that when hematite particles aggregates having an average length of 0.005 to 0.6 xcexcm and an average width of 0.001 to 0.40 xcexcm, which are obtained by subjecting acicular goethite particles to milling treatment and then heat-dehydrating the treated particles at a temperature of 200 to 540, are used for forming a non-magnetic undercoat layer for magnetic recording medium, the obtained non-magnetic undercoat layer can be considerably improved in surface smoothness. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide hematite particles aggregates as non-magnetic particles capable of forming a non-magnetic undercoat layer for magnetic recording medium having a more excellent surface smoothness, which can exhibit not only an excellent dispersibility but also have an improving ability of a surface smoothness of a coating film by calendering treatment.
Another object of the present invention is to provide a non-magnetic undercoat layer for magnetic recording medium having a more excellent surface smoothness, which contains the above hematite particles aggregates.
A further object of the present invention is to provide a magnetic recording medium having the non-magnetic undercoat layer, exhibiting a more excellent surface smoothness.
To accomplish the aims, in a first aspect of the present invention, there are provided hematite particles aggregates comprising aggregates of acicular hematite particles oriented in a major axis direction thereof, said acicular hematite particles having an average major axis diameter of 0.005 to 0.3 xcexcm and an average minor axis diameter of 0.0005 to 0.10 xcexcm,
said hematite particles aggregates exhibiting a compressiblity of a coating film of 9.0 to 20.0%, when measured by the following method:
(i) 12 g of the hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70 by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having solid content of 72%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material;
(ii) the obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, are charged into a 140-ml glass bottle at the following mixing ratio; and a resultant mixture is mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a non-magnetic coating material:
(iii) then, the obtained non-magnetic coating material is applied onto a non-magnetic base film using an applicator with a coating thickness of 55 xcexcm and then dried, thereby forming a non-magnetic undercoat layer; and
(iv) the thus obtained dried non-magnetic undercoat layer is subjected to calendering treatment at 85xc2x0 C. under a load of 200 kg/cm four times, and the compressiblity of the coating film is calculated from thicknesses to (xcexcm) and t1 (xcexcm) of the coating film before and after the calendering treatment, respectively, according to the following formula:
Compressiblity of coating film (%)={(t0xe2x88x92t1)/t0}xc3x97100 
wherein t0 represents a thickness of the non-magnetic undercoat layer before the calendering treatment; and t1 represents a thickness of the non-magnetic undercoat layer after the calendering treatment.
In a second aspect of the present invention, there are provided hematite particles aggregates comprising aggregates of acicular hematite particles oriented in a major axis direction thereof, said acicular hematite particles having an average major axis diameter of 0.005 to 0.3 xcexcm and an average minor axis diameter of 0.0005 to 0.10 xcexcm,
said hematite particles aggregates exhibiting a specific surface area value of 100 to 250 m2/g, a cyclohexanone absorption of not less than 0.6 ml/g, a soluble sodium salt content of not more than 300 ppm calculated as Na, a soluble sulfate content of not more than 150 ppm calculated as SO4, and a compressiblity of coating film of 9.0 to 20.0% when measured by the following method:
(i) 12 g of the hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having a solid content of 72%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material;
(ii) the obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, are charged into a 140-ml glass bottle at the following mixing ratio; and a resultant mixture is mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a non-magnetic coating material:
(iii) then, the obtained non-magnetic coating material is applied onto a non-magnetic base film using an applicator with a coating thickness of 55 xcexcm and then dried, thereby forming a non-magnetic undercoat layer; and
(iv) the thus obtained dried non-magnetic undercoat layer is subjected to calendering treatment at 85xc2x0 C. under a load of 200 kg/cm four times, and the compressiblity of the coating film is calculated from thicknesses to (xcexcm) and t1 (xcexcm) of the coating film before and after the calendering treatment, respectively, according to the following formula:
Compressiblity of coating film (%)={(t0xe2x88x92t1)/t0}xc3x97100 
wherein t0 represents a thickness of the non-magnetic undercoat layer before the calendering treatment; and t1 represents a thickness of the non-magnetic undercoat layer after the calendering treatment.
In a third aspect of the present invention, there are provided hematite particles aggregates comprising aggregates of acicular hematite particles oriented in a major axis direction thereof, said acicular hematite particles having an average major axis diameter of 0.005 to 0.3 xcexcm and an average minor axis diameter of 0.0005 to 0.10 xcexcm.
said hematite particles aggregates having a surface-coating layer composed of at least one selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, and having a specific surface area value of 100 to 250 m2/g, a cyclohexanone absorption of not less than 0.6 ml/g, a soluble sodium salt content of not more than 300 ppm calculated as Na, a soluble sulfate content of not more than 150 ppm calculated as S04, and a compressiblity of coating film of 9.0 to 20.0% when measured by the following method:
(i) 12 g of the hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having a solid content of 72%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material;
(ii) the obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, are charged into a 140-ml glass bottle at the following mixing ratio; and a resultant mixture is mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a non-magnetic coating material:
(iii) then, the obtained non-magnetic coating material is applied onto a non-magnetic base film using an applicator with a coating thickness of 55 xcexcm and then dried, thereby forming a non-magnetic undercoat layer; and
(iv) the thus obtained non-magnetic undercoat layer is subjected to calendering treatment at 85xc2x0 C. under a load of 200 kg/cm four times, and the compressiblity of the coating film is calculated from thicknesses t0 (xcexcm) and t1 (xcexcm) of the coating film before and after the calendering treatment, respectively, according to the following formula:
Compressiblity of coating film (%)={(t0xe2x88x92t1)/t0}xc3x97100 
wherein t0 represents a thickness of the non-magnetic undercoat layer before the calendering treatment; and t1 represents a thickness of the non-magnetic undercoat layer after the calendering treatment.
In a fourth aspect of the present invention, there is provided a non-magnetic base film for a magnetic recording medium comprising a non-magnetic base film, and a non-magnetic undercoat layer formed on the non-magnetic substrate which comprises the hematite particles aggregates as defined in the first aspect, and a binder resin.
In a fifth aspect of the present invention, there is provided a non-magnetic substrate for a magnetic recording medium comprising a non-magnetic base, and a non-magnetic undercoat layer formed on the non-magnetic base film which comprises the hematite particles aggregates as defined in the first aspect, and a binder resin, and exhibits a gloss of 180 to 300%, a surface roughness Ra of coating film of 0.5 to 8.0 nm, and a compressiblity of coating film of 9.0 to 20.0% when measured by the following method:
(i) 12 g of the hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having a solid content of 72%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material;
(ii) the obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, are charged into a 140-ml glass bottle at the following mixing ratio; and a resultant mixture is mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a non-magnetic coating material:
(iii) then, the obtained non-magnetic coating material is applied onto a non-magnetic base film using an applicator with a coating thickness of 55 xcexcm and then dried, thereby forming a non-magnetic undercoat layer; and
(iv) the thus obtained non-magnetic undercoat layer is subjected to calendering treatment at 85xc2x0 C. under a load of 200 kg/cm four times, and the compressiblity of the coating film is calculated from thicknesses t0 (xcexcm) and t1 (xcexcm) of the coating film before and after the calendering treatment, respectively, according to the following formula:
Compressiblity of coating film (%)={(t0xe2x88x92t1)/t0}xc3x97100 
wherein t0 represents a thickness of the non-magnetic undercoat layer before the calendering treatment; and t1 represents a thickness of the non-magnetic undercoat layer after the calendering treatment.
In a six aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic base film, a non-magnetic undercoat layer formed on the non-magnetic base film which comprises the hematite particles aggregates as defined in the first aspect, and a binder resin, and a magnetic recording layer formed on the non-magnetic undercoat layer which comprises magnetic particles and a binder resin.
In a seventh aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic base film, a non-magnetic undercoat layer formed on the non-magnetic base film which comprises the hematite particles aggregates as defined in the first aspect, and a binder resin; and a magnetic recording layer formed on the non-magnetic undercoat layer which comprises magnetic particles and a binder resin, and
having a coercive force value of 39.8 to 318.3 kA/m (500 to 4,000 Oe), a squareness (residual magnetic Flux density Br/saturation magnetic flux density Bm) of 0.85 to 0.95, a gloss of coating film of 170 to 300%, a surface roughness Ra of coating film of not more than 8.5 nm, and a compressiblity of 7.5 to 19.0% when measured by the following method:
(i) 12 g of the hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having a solid content of 72%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material;
(ii) the obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, are charged into a 140-ml glass bottle at the following mixing ratio; and a resultant mixture is mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a non-magnetic coating material:
(iii) then, the obtained non-magnetic coating material is applied onto a non-magnetic base film using an applicator with a coating thickness of 55 xcexcm and then dried, thereby forming a non-magnetic undercoat layer;
(iv) 12 g of magnetic particles are mixed with fine carbon black particles, alumina particles as an abrasive, a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having a solid content of 78%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material;
(v) the obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, 35% by weight of toluene and 35% by weight of methyl ethyl ketone), cyclohexanone, toluene and methyl ethyl ketone, are charged into a 140-ml glass bottle at the following mixing ratio; a resultant mixture is mixed and dispersed for 6 hours using a paint shaker to obtain a coating material; the obtained coating material is further mixed with a lubricant and a curing agent: and a resultant mixture is mixed and dispersed for 15 minutes using a paint shaker, thereby obtaining a magnetic coating material:
(vi) the obtained magnetic coating material is applied onto the non-magnetic undercoat layer formed on the non-magnetic base film using an applicator with a coating thickness of 15 xcexcm and a resultant coating film is oriented in a magnetic field and then dried, thereby obtaining a magnetic recording layer; and
(vii) the thus obtained coating film is subjected to calendering treatment at 85xc2x0 C. under a load of 200 kg/cm four times, and the compressiblity of the coating film is calculated from thicknesses t2 (xcexcm) and t3 (xcexcm) of the coating film before and after the calendering treatment, respectively, according to the following formula:
Compressiblity of coating film (%)={(t2xe2x88x92t3)/t2}xc3x97100 
wherein t2 represents a total thickness of the non-magnetic undercoat layer and the magnetic recording layer before the calendering treatment; and t3 represents a total thickness of the non-magnetic undercoat layer and the magnetic recording layer after the calendering treatment.
The present invention will be described in detail below.
First, the hematite particles aggregates for non-magnetic undercoat layer according to the present invention are described.
The hematite particles aggregates for non-magnetic undercoat layer according to the present invention are constituted by usually plural number of acicular hematite particles oriented along the major axis direction thereof.
The hematite particles aggregates of the present invention have an average length of usually 0.005 to 0.6 xcexcm, preferably 0.01 to 0.45 xcexcm, more preferably 0.02 to 0.3 xcexcm. When the average length is more than 0.6 xcexcm, the size of the hematite particles aggregates is too large. When such large particles are used for forming a non-magnetic undercoat layer, the obtained coating film may tend to be deteriorated in surface smoothness. When the average length is less than 0.005 xcexcm, such particles may tend to be agglomerated together because of the increase in intermolecular force due to fine particles, resulting in poor dispersibility in vehicle upon production of a non-magnetic coating material.
The hematite particles aggregates of the present invention have an average width of usually 0.001 to 0.40 xcexcm, preferably 0.002 to 0.30 xcexcm, more preferably 0.004 to 0.20 xcexcm. When the average width is less than 0.001 xcexcm, such particles may tend to be agglomerated together because of the increase in intermolecular force due to fine particles, resulting in poor dispersibility in vehicle upon production of a non-magnetic coating material.
The hematite particles aggregates of the present invention have a ratio of average length to average width of usually 1.5:1 to 15:1, preferably 2.0:1 to 12.5:1, more preferably 2.5:1 to 10:1. When the ratio of average length to average width is more than 15:1, such particles may tend to be entangled together, resulting in poor dispersibility in vehicle upon production of a non-magnetic coating material, or to increase in viscosity thereof. When the ratio of average length to average width is less than 1.5:1, such particles may tend to be deteriorated in dispersibility in vehicle upon production of a non-magnetic coating material.
The hematite particles aggregates of the present invention have a compressiblity of coating film of usually 9.0 to 20.0%, preferably 9.5 to 20.0%, more preferably 11.0 to 20.0% as measured with respect to a non-magnetic undercoat layer produced therefrom by the following method.
(i) 12 g of hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having solid content of 72%. The resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material.
(ii) The obtained kneaded material together with 95 g of 1.5 mmxcfx86 glass beads, an additional binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, were charged into a 140-ml glass bottle at the following mixing ratio. The resultant mixture was mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a magnetic coating material.
(iii) The obtained non-magnetic coating material was applied onto a non-magnetic base film using an applicator with a coating thickness of 55 xcexcm and then dried, thereby forming a non-magnetic undercoat layer.
(iv) The thus obtained non-magnetic undercoat layer was subjected to calendering treatment at 85xc2x0 C. under a load of 200 kg/cm four times. The compressiblity of the coating film was calculated from thicknesses t0 (xcexcm) and t1 (xcexcm) of the coating film before and after the calendering treatment, respectively, according to the following formula:
Compressiblity of coating film (%)={(t0xe2x88x92t1)/t0}xc3x97100 
wherein t0 represents a thickness of a non-magnetic undercoat layer before calendering treatment; and t1 represents a thickness of a non-magnetic undercoat layer after calendering treatment.
When the non-magnetic undercoat layer has a compressiblity of coating film of less than 9.0%, it may be difficult to obtain the effect of sufficiently improving a surface smoothness of the coating film by calendering treatment. When the non-magnetic undercoat layer has a compressiblity of coating film of more than 20.0%, the thickness of the coating film is considerably fluctuated, so that it becomes difficult to design a suitable magnetic recording medium.
The hematite particles aggregates of the present invention have a BET specific surface area value of preferably 100 to 250 m2/g. When the BET specific surface area value is more than 250 m2/g, the packing condition of the particles contained in the coating film may become too dense, so that it may be difficult to attain a good surface-smoothing effect by the calendering treatment. When the BET specific surface area value is less than 100 m2/g, the obtained particles may become too coarse to sufficiently improve a surface smoothness of the coating film. In the consideration of surface smoothness of the obtained magnetic recording medium, the BET specific surface area value of the hematite particles aggregates is more preferably 100 to 225 m2/g, still more preferably 110.9 to 200 m2/g.
The hematite particles aggregates of the present invention have a cyclohexanone absorption of preferably not less than 0.6 ml/g, more preferably 0.65 to 1.5 ml/g. When the cyclohexanone absorption is less than 0.6 ml/g, it is suggested that the aggregated particles may fail to have the structure as specified in the present invention. Therefore, it may be difficult to obtain the effect of improving a surface smoothness of a coating film by calendering treatment.
In particular, in the consideration of corrosion resistance of the obtained magnetic recording medium, the hematite particles aggregates of the present invention are preferably composed of such high-purity hematite particles having a less soluble sodium salt content, a less soluble sulfate content or the like.
Specifically, the high-purity hematite particles aggregates have a soluble sodium salt content of usually not more than 300 ppm, preferably not more than 200 ppm, calculated as Na; and a soluble sulfate content of usually not more than 150 ppm, preferably not more than 100 ppm, calculated as SO4.
Also, the hematite particles aggregates of the present invention preferably have a pH value of usually not less than 8.0, preferably 8.2 to 11.0.
The hematite particles aggregates of the present invention may be coated, if required, with at least one surface-coating material selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. The thus surface-coated hematite particles aggregates can show a good compatibility with a binder resin when dispersed in vehicle, thereby readily attaining the aimed dispersibility.
The amount of the surface-coating material applied, i.e., a hydroxides and/or oxides of aluminum and/or silicon coat, is preferably 0.01 to 50% by weight, calculated as Al for hydroxides of aluminum or oxides of aluminum, and calculated as SiO2 for hydroxides of silicon or oxides of silicon, base on the weight of the hematite particles aggregates. When the amount of the surface-coating material applied is less than 0.01% by weight, the effect of improving the dispersibility can not achieved. When the amount of the surface-coating material applied is more than 50% by weight, the effect obtained by the coating is already saturated and, therefore, it is unnecessary and meaningless to use such a large coating amount. In the consideration of the effect of improving the dispersibility and industrial productivity, the amount of the surface-coating material applied is more preferably 0.05 to 20% by weight.
In the case where the aluminum and silicon compounds are used in combination, the amount of the surface-coating material applied is preferably 0.01 to 50% by weight, more preferably 0.05 to 20% by weight (calculated as a sum of Al and SiO2) based on the weight of the hematite particles aggregates.
The hematite particles aggregates constituted by hematite particles coated with the surface-coating material have substantially the same particle length, particle width, ratio of average length to average width and BET specific surface area value as those of the hematite particles aggregates constituted by hematite particles uncoated with the surface-coating material.
The acicular hematite particles used in the present invention have an average major axis diameter of 0.005 to 0.3 xcexcm, preferably 0.008 to 0.25 xcexcm, more preferably 0.01 to 0.2 xcexcm.
When the average major axis diameter of the acicular hematite particles is more than 0.3 xcexcm, the size of the obtained particles may become too large. As a result, when such large particles are used, the obtained non-magnetic undercoat layer may tend to be deteriorated in surface smoothness. When the average major axis diameter of the acicular hematite particles is less than 0.005 xcexcm, such particles may tend to be agglomerated together because of the increase in intermolecular force due to fine particles, thereby failing to obtain aggregated particles oriented along the same direction.
The acicular hematite particles used in the present invention have an average minor axis diameter of 0.0005 to 0.1 xcexcm, preferably 0.0006 to 0.05 xcexcm, more preferably 0.0007 to 0.02 xcexcm.
When the average minor axis diameter of the acicular hematite particles is less than 0.0005 xcexcm, such particles may tend to be agglomerated together because of the increase in intermolecular force due to fine particles, thereby failing to obtain aggregated particles oriented along the major axis direction. The acicular hematite particles having an average minor axis diameter of more than 0.1 xcexcm are difficult to industrially produce.
The acicular hematite particles used in the present invention have a ratio of an average major axis diameter to an average minor axis diameter (hereinafter referred to merely as xe2x80x9caspect ratioxe2x80x9d) of 3:1 to 30:1, preferably 5:1 to 28:1, more preferably 10:1 to 25:1.
When the aspect ratio is less than 3:1 or more than 30:1, the obtained non-magnetic undercoat layer exhibits a low strength.
The acicular hematite particles used in the present invention may contain aluminum inside thereof. By using the acicular hematite particles containing aluminum inside thereof, the obtained magnetic recording medium can exhibit an improved durability. The amount of aluminum contained inside of the acicular hematite particles is preferably 0.05 to 50% by weight, more preferably 0.05 to 40% by weight, calculated as Al.
Next, the magnetic recording medium of the present invention is described.
The magnetic recording medium of the present invention comprises a non-magnetic base film, a non-magnetic undercoat layer formed on the non-magnetic base film, and a magnetic recording layer formed on the non-magnetic undercoat layer.
As the non-magnetic base film, there may be used those presently used in ordinary magnetic recording media, e.g., films of synthetic resins such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonates, polyethylene naphthalate, polyamides, polyamideimides and polyimides, foils and plates of metals such as aluminum and stainless steel, and various papers. The thickness of the non-magnetic base film may be varied depending upon materials thereof, and is preferably 1.0 to 300 xcexcm, more preferably 2.0 to 200 xcexcm.
More specifically, in the case of magnetic discs, a non-magnetic base film thereof may be usually made of polyethylene terephthalate, and has a thickness of usually 50 to 300 xcexcm, preferably 60 to 200 xcexcm. In the case of magnetic tapes, a non-magnetic base film thereof may be made of polyethylene terephthalate, polyethylene naphthalate, polyamides or the like; and the polyethylene terephthalate for magnetic tapes has a thickness of usually 3 to 100 xcexcm, preferably 4 to 20 xcexcm; the polyethylene naphthalate has a thickness of usually 3 to 50 xcexcm, preferably 4 to 20 xcexcm; and the polyamide has a thickness of usually 2 to 10 xcexcm, preferably 3 to 7 xcexcm.
The non-magnetic undercoat layer of the present invention comprises a binder resin, and the hematite particles aggregates of the present invention, or the hematite particles aggregates coated with the surface-coating material according to the present invention.
As the binder, there may be used those presently used for the production of ordinary magnetic recording media, e.g., vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivatives such as nitrocellulose, etc., polyester resin, synthetic rubber-based resin such as polybutadiene, etc., epoxy resin, polyamide resin, polyisocyanate, electron beam-curable acrylic urethane resin, or mixtures thereof. Also, the binder resin may contain polar groups such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94OPO2M2 and xe2x80x94NH2 wherein M represents hydrogen, sodium or potassium. In the consideration of dispersibility of the hematite particles aggregates in vehicle, it is preferred to use the binder resins containing xe2x80x94COOH or xe2x80x94SO3M as a polar group.
The amount of the hematite particles aggregates contained in the non-magnetic undercoat layer is usually 5 to 2,000 parts by weight, preferably 100 to 1,000 parts by weight based on 100 parts by weight of the binder resin.
When the amount of the hematite particles aggregates used is less than 5 parts by weight, the amount of the hematite particles aggregates contained in a non-magnetic coating material is too small. As a result, upon producing a coating film from such a non-magnetic coating material, it may be difficult to obtain a layer containing the hematite particles aggregates continuously dispersed therein, resulting in insufficient surface smoothness and strength of the obtained coating film. When the amount of the hematite particles aggregates used is more than 2,000 parts by weight, the amount of the hematite particles aggregates used may be too large as compared to the amount of the binder resin used. As a result, it may be difficult to sufficiently disperse the hematite particles aggregates in the non-magnetic coating material, thereby failing to obtain a coating film having a sufficient surface smoothness. Further, the hematite particles aggregates may not be sufficiently bonded together by the binder resin, thereby failing to obtain a stable coating film.
The thickness of the non-magnetic undercoat layer formed on the non-magnetic base film is preferably 0.2 to 10 xcexcm. When the thickness of the non-magnetic undercoat layer is less than 0.2 xcexcm, it may be difficult to improve the surface roughness of the non-magnetic base film, and the obtained coating film may tend to become insufficient in strength. In the consideration of reduction in thickness of the obtained magnetic recording medium and strength of the coating film, the thickness of the non-magnetic undercoat layer is more preferably 0.5 to 5 xcexcm.
Meanwhile, the non-magnetic undercoat layer may further contain various additives used for the production of ordinary magnetic recording media such as lubricants, abrasives, anti-static agents and the like.
The non-magnetic undercoat layer using the hematite particles aggregates according to the present invention has a gloss of coating film of usually 180 to 300%, and a surface roughness Ra of coating film of usually 0.5 to 8.0 nm. As to the strength of the coating film, the non-magnetic undercoat layer has a Young""s modulus (relative value) of usually 126 to 160, and a compressiblity of coating film of usually of 9.0 to 20.0%, preferably 11.0 to 20.0%.
The non-magnetic undercoat layer using the hematite particles aggregates uncoated with the above surface-coating material according to the present invention has a gloss of coating film of usually 180 to 300%, preferably 185 to 300%, and a surface roughness Ra of coating film of usually 0.5 to 8.0 nm, preferably 0.5 to 7.5 nm. As to the strength of the coating film, the non-magnetic undercoat layer has a Young""s modulus (relative value) of 126 to 160, preferably 128 to 160, and a compressiblity of coating film of usually 9.0 to 20.0%, preferably 9.5 to 19.0%, more preferably 10.0 to 18.0%.
The non-magnetic undercoat layer using the hematite particles aggregates coated with the above surface-coating material according to the present invention has a gloss of coating film of usually 185 to 300%, preferably 190 to 300%, and a surface roughness Ra of coating film of usually 0.5 to 7.5 nm, preferably 0.5 to 7.0 nm. As to the strength of the coating film, the non-magnetic undercoat layer has a Young""s modulus (relative value) of usually 128 to 160, preferably 130 to 160, and a compressiblity of coating film of usually 9.5 to 20.0%, preferably 10.0 to 19.0%, more preferably 10.5 to 18.0%.
The magnetic recording medium of the present invention Comprises magnetic particles and a binder resin.
As the magnetic particles, there may be used cobalt-coated magnetic iron oxide particles obtained by coating with Co, or Co and Fe, on magnetic iron oxide particles such as maghemite particles (xcex3-Fe2O3); and magnetite particles (FeOx.Fe2O3, 0 less than xxe2x89xa61), cobalt-coated magnetic iron oxide particles obtained by incorporating other elements than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals into the above cobalt-coated magnetic iron oxide particles; acicular magnetic metal particles containing iron as a main component; acicular magnetic iron alloy particles containing other elements than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals; magnetoplumbite-type plate-shaped ferrite particles containing Ba, Sr or Baxe2x80x94Sr; magnetoplumbite-type plate-shaped ferrite particles obtained by incorporating into the above magnetoplumbite-type plate-shaped ferrite particles, one or more coercive force reducing agents selected from the group consisting of divalent and tetravalent metals such as Co, Ni, Zn, Mn, Mg, Ti, Sn, Zr, Nb, Cu and Mo, or the like.
In the consideration of recent short-wavelength recording and high-density recording, among the above magnetic particles, acicular magnetic metal particles containing iron as a main component, and acicular magnetic iron alloy particles containing other elements than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals, are preferred.
The magnetic particles have an average major axis diameter (average plate surface diameter in the case of plate-shaped particles) of usually 0.01 to 0.5 xcexcm preferably 0.03 to 0.3 xcexcm. The magnetic particles are preferably acicular particles or plate-shaped particles. Here, the xe2x80x9cacicularxe2x80x9d shape means in addition to literally an acicular shape, a spindle shape and a rice-ball shape.
The acicular magnetic particles have an aspect ratio of usually not less than 3:1, preferably not less than 5:1. In the consideration of dispersibility in vehicle, the upper limit of the aspect ratio of the acicular magnetic particles is usually 15:1, preferably 10:1.
The plate-shaped magnetic particles have a ratio of an average plate surface diameter to an average thickness (hereinafter referred to merely as xe2x80x9cplate ratioxe2x80x9d) of usually not less than 2:1, preferably not less than 3:1. In the consideration of dispersibility in vehicle, the upper limit of the plate ratio of the plate-shaped magnetic particles is usually 50:1, preferably 45:1.
As to the magnetic properties of the magnetic particles, the coercive force value thereof is usually 39.8 to 318.3 kA/m (500 to 4,000 Oe), preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); and the saturation magnetization value thereof is usually 50 to 170 Am2/kg (50 to 170 emu/g), preferably 60 to 170 Am2/kg (60 to 170 emu/g).
In the consideration of high-density recording, etc., the it is preferred to use the acicular magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles. As to the magnetic properties of the acicular magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles, the coercive force value thereof is usually 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); and the saturation magnetization value thereof is usually 90 to 170 Am2/kg (90 to 170 emu/g), preferably 100 to 170 Am2/kg (100 to 170 emu/g).
As the binder resin for the magnetic recording layer, there may be used the same binder resins as used for Forming the above non-magnetic undercoat layer.
The thickness of the magnetic recording layer formed on the non-magnetic undercoat layer is usually 0.01 to 5 xcexcm, preferably 0.05 to 1 xcexcm. When the thickness of the magnetic recording layer is less than 0.01 xcexcm, it may be difficult to form a uniform coating film, resulting in problems such as coating unevenness or the like. When the thickness of the magnetic recording layer is more than 5 xcexcm, the obtained magnetic recording layer may fail to show the aimed electromagnetic performance because of influence of demagnetizing field.
The amount of the magnetic particles contained in the magnetic recording layer is usually 200 to 2,000 parts by weight, preferably 300 to 1,500 parts by weight based on 100 parts by weight of the binder resin.
The magnetic recording layer may further contain ordinarily used additives such as lubricants, abrasives, anti-static agents or the like.
The magnetic recording medium of the present invention has a coercive force value of 39.8 to 318.3 kA/m (500 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95; a gloss of coating film of usually 170 to 300%; a surface roughness Ra of a coating film of usually not more than 8.5 nm; a Young""s modulus (relative value) of usually 128 to 160; and a compressiblity of coating film of usually 7.5 to 19.0% when as measured with a coating film comprising the non-magnetic undercoat layer and the magnetic layer.
The magnetic recording medium produced using the above magnetic particles as magnetic particles for magnetic recording layer, and the hematite particles aggregates uncoated with the surface-coating material as non-magnetic particles for non-magnetic undercoat layer according to the present invention, has a coercive force value of 39.8 to 318.3 kA/m (500 to 4,000 Oe), preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 170 to 300%, preferably 175 to 300%; a surface roughness Ra of a coating film of usually not more than 8.5 nm, preferably 2.0 to 8.0 nm; a Young""s modulus (relative value) of usually 128 to 160, preferably 130 to 160; and a compressiblity of coating film of usually 7.5 to 19.0%, preferably 8.0 to 18.0%, more preferably 8.5 to 17.0% when as measured with a coating film comprising the non-magnetic undercoat layer and the magnetic layer.
The magnetic recording medium produced using the above magnetic for magnetic recording layer, and the hematite particles aggregates coated with the surface-coating material as non-magnetic particles for non-magnetic undercoat layer according to the present invention, has a coercive force value of 39.8 to 318.3 kA/m (500 to 4,000 Oe), preferably 43.8 to 318.3 kA/m (550 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 175 to 300%, preferably 180 to 300%; a surface roughness Ra of a coating film of usually not more than 8.0 nm, preferably 2.0 to 7.5 nm; a Young""s modulus (relative value) of usually 130 to 160, preferably 132 to 160; and a compressiblity of coating film of usually 8.0 to 19.0%, preferably 8.5 to 18.0%, more preferably 9.0 to 17.0% when as measured with a coating film comprising the non-magnetic undercoat layer and the magnetic layer.
In the consideration of high-density recording, etc. of the magnetic recording medium, it is suitable that the acicular magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles are used as magnetic particles for magnetic recording layer, and the hematite particles aggregates uncoated with the surface-coating material are used as non-magnetic particles for non-magnetic undercoat layer. Such a magnetic recording medium has a coercive force value of 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.87 to 0.95, preferably 0.88 to 0.95; a gloss of coating film of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra of a coating film of usually not more than 7.5 nm, preferably 2.0 to 7.5 nm; a Young""s modulus (relative value) of usually 128 to 160, preferably 130 to 160; and a compressiblity of coating film of usually 7.5 to 19.0%, preferably 8.0 to 18.0%, more preferably 8.5 to 17.0% when as measured with a coating film comprising the non-magnetic undercoat layer and the magnetic layer.
The magnetic recording medium produced by using the acicular magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles as magnetic particles for magnetic recording layer, and the hematite particles aggregates coated with the surface-coating material as non-magnetic particles for non-magnetic undercoat layer, has a coercive force value of 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.87 to 0.95, preferably 0.88 to 0.95; a gloss of coating film of usually 205 to 300%, preferably 210 to 300%; a surface roughness Ra of a coating film of usually not more than 7.0 nm, preferably 2.0 to 6.5 nm; a Young""s modulus (relative value) of usually 130 to 160, preferably 132 to 160; and a compressiblity of coating film of usually 8.0 to 19.0%, preferably 8.5 to 18.0%, more preferably 9.0 to 17.0% when as measured with a coating film comprising the non-magnetic undercoat layer and the magnetic layer.
In addition, the magnetic recording medium produced using the hematite particles aggregates composed of acicular hematite particles containing aluminum inside thereof according to the present invention as non-magnetic particles for non-magnetic undercoat layer, can be improved in durability. Specifically, as to the durability of such a magnetic recording medium, the running durability thereof is usually not less than 20 minutes, preferably not less than 22 minutes; and the scratch resistance thereof is usually rank A or B, preferably rank A.
In particular, in the case of the magnetic recording medium produced by using the acicular magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles as magnetic particles for magnetic recording layer, and the high-purity hematite particles aggregates according to the present invention as non-magnetic particles for non-magnetic undercoat layer, the corrosion resistance as represented by the change percentage (%) of coercive force value of the magnetic recording medium thereof is usually not more than 10.0%, preferably not more than 9.5%; and the corrosion resistance as represented by the change percentage (%) of saturation magnetization thereof is usually not more than 10.0%, preferably not more than 9.5%.
Next, the process for producing the hematite particles aggregates according to the present invention, is described.
The hematite particles aggregates according to the present invention can be produced by subjecting acicular goethite particles to milling treatment, and then heat-dehydrating the treated acicular goethite particles at a temperature of usually 200 to 540xc2x0 C.
The acicular goethite particles used in the present invention can be produced by passing an oxygen-containing gas such as air through a suspension containing an iron hydroxide obtained by reacting ferrous salt with an aqueous alkali hydroxide solution.
The acicular goethite particles used in the present invention preferably have an average major axis diameter of usually 0.005 to 0.35 xcexcm, an average minor axis diameter of usually 0.0005 to 0.12 xcexcm, an aspect ratio of usually 3:1 to 30:1, and a BET specific surface area value of usually 100 to 250 m2/g.
Meanwhile, the surface of the acicular goethite particles used in the present invention may be coated, if required, with an anti-sintering agent containing P, Si, B, Zr, Sb or the like.
Upon the above milling treatment of the acicular goethite particles, there may be used a water suspension obtained by subjecting a suspension containing the goethite particles produced by the goethite production reaction to filtering-out and water-washing, and then dispersing again the goethite particles wet-cake in water; or a water suspension obtained by subjecting a suspension containing goethite particles produced by the goethite production reaction to filtering-out, water-washing and drying to taken out goethite particles, and then dispersing again the goethite particles in water. Among these water suspensions, it is preferred to use the water suspension obtained by dispersing again the goethite particles wet-cake in water.
The milling treatment of the acicular goethite particles may be conducted by adjusting the concentration of the slurry containing the acicular goethite particles to usually 30 to 500 g/liter, preferably 40 to 250 g/liter, more preferably 50 to 200 g/liter, and then subjecting the obtained slurry to milling treatment by applying a shear force of usually 1,000 to 9,000 rpm, preferably 1,200 to 5,000 rpm thereto. This milling treatment causes the acicular goethite particles to orient in the major axis direction thereof.
As apparatuses usable for subjecting the acicular goethite particles to milling treatment, there may be used those apparatuses capable of applying a suitable shear force to the slurry, for example, wet-grinding apparatuses such as grinders and ultrafine-pulverizers.
Specific examples of the wet-grinding apparatuses may include SUPER MASUCOLLOTIDER and SELENDEPUTER (manufactured by Masuko Sangyo Co., Ltd.), T.K. MYCOLLOIDER (manufactured by Tokushu Kika Kogyo Co., Ltd.) or the like.
Then, the treated acicular goethite particles are heat-treated at a temperature of usually 200 to 540xc2x0 C., preferably 250 to 500xc2x0 C., more preferably 280 to 450xc2x0 C., thereby obtaining aggregates of the acicular hematite particles. When the heat-treating temperature is less than 200xc2x0 C., it may take a too long period of time until the dehydration reaction is completed. When the heat-treating temperature is more than 540xc2x0 C., the dehydration reaction may abruptly proceed, resulting in destruction of shape of the obtained particles, or the obtained particles may suffer from sintering therebetween. Also, the heat-treating time is preferably in the range of 30 minutes to 3 hours.
The high-purity hematite particles aggregates can be obtained by further heat-treating the aggregates of hematite particles obtained after the above milling treatment and heat-treatment, in an aqueous alkali solution, and then filtering out and water-washing the heat-treated particles.
The pH value of the aqueous alkali solution is preferably not less than 13.0, and the heat-treating temperature is preferably not less than 80xc2x0 C., more preferably not less than 90xc2x0 C.
The hematite particles aggregates coated with the surface-coating material according to the present invention, can be produced by (i) adding an aluminum compound and/or a silicon compound to a water suspension containing the aggregates of hematite particles obtained after the above milling treatment and heat-treatment, and then mixing the resultant mixture under stirring, and, if required, further adjusting the pH value of the mixture obtained after the mixing, thereby coating the surface of the hematite particles aggregates with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, and then (ii) subjecting the obtained coated hematite particles aggregates to filtering-out, water-washing, drying and pulverizing treatments. The thus obtained hematite particles aggregates may be further subjected to deaeration, compaction or other treatments.
Examples of the aluminum compound may include aluminum salts such as aluminum acetate, aluminum sulfate, aluminum, chloride and aluminum nitrate; alkali aluminates such as sodium aluminate; or the like. Examples of the silicon compound may include water glass #3, sodium orthosilicate, sodium metasilicate or the like.
Next, the process for producing the magnetic recording medium of the present invention is described.
The magnetic recording medium of the present invention can be produced by ordinary methods, i.e., by applying a non-magnetic coating material containing the hematite particles aggregates, a binder resin and a solvent onto a non-magnetic base film, and drying the obtained coating layer, thereby forming a non-magnetic undercoat layer; applying a magnetic coating material containing magnetic particles, a binder resin and a solvent onto the thus obtained non-magnetic undercoat layer, thereby forming a magnetic recording layer; and then subjecting the thus formed magnetic recording layer to magnetic orientation, subjecting to calendering treatment and then curing.
As the solvent used for forming the non-magnetic undercoat layer or the magnetic recording layer, there may be exemplified those solvents ordinarily used for production of magnetic recording media, such as methyl ethyl ketone, toluene, cyclohexane, methyl isobutyl ketone, tetrahydroturan and mixtures thereof.
The total amount of the solvent used is 50 to 1,000 parts by weight based on 100 parts by weight of the hematite particles aggregates or magnetic particles. When the amount of the solvent used is less than 50 parts by weight, the obtained coating material may exhibit a too high viscosity, so that it may be difficult to coat such a high-viscous coating material. When the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent vaporized from the coating material may be too large and, therefore, disadvantageous from industrial viewpoint.
The point of the present invention is that the magnetic recording medium having a non-magnetic undercoat layer using the hematite particles aggregates of the present invention can exhibit an excellent surface smoothness.
The reason why the magnetic recording medium having a non-magnetic undercoat layer using the hematite particles aggregates of the present invention can exhibit an excellent surface smoothness, is considered as follows, though not clearly determined yet.
That is, since the acicular hematite particles used in the present invention are fine particles, such fine particles are usually difficult to disperse in vehicle because these particles tend to be agglomerated together by the increase in intermolecular force due to the fine particles. On the contrary, in the case of the hematite particles aggregates of the present invention, the raw acicular goethite particles are previously oriented in the major axis direction thereof and then heat-treated at such a temperature that the particles are free from sintering therebetween and destruction of the oriented condition. As a result, the thus obtained hematite particles aggregates are constituted by the acicular hematite particles oriented in the major axis direction thereof, and can maintain the oriented condition until the calendering treatment of the coating film. For this reason, it is considered that such hematite particles aggregates can be readily dispersed in vehicle because of large particle size in vehicle, and can be compressed in the thickness direction of the coating film and readily oriented in the same direction therein by subjecting the coating film to calendering treatment, thereby obtaining a magnetic recording medium having a more excellent surface smoothness.
Thus, when the hematite particles aggregates for non-magnetic undercoat layer according to the present invention are used, it is possible to obtain a non-magnetic undercoat layer having an excellent surface smoothness. Therefore, when such a non-magnetic undercoat layer is used, it is possible to produce a magnetic recording medium capable of exhibiting an excellent surface smoothness.
Further, the magnetic recording medium of the present invention can exhibit an excellent surface smoothness as described above and, therefore is suitable as a high-density magnetic recording medium.